You know, I hear people say that left and right, but I've NEVER experienced that.

I've had it happen, it was weird.

Honestly if I'm moving 2, 2 1/2, or 3 globals, I'll use a Plague > Death > Plague, prevents it from happening again, and I don't burn too much mana. Feeling the lost Shadowfiend cooldown of 2pc12 kind of hurts.

Well... If you hit DP and immediately hit it again (don't wait for the GCD but use the queue so it casts again as soon as the GCD is up) you generally will get the more powerful spell error because the first tick of the spell hasn't happened yet. If you wait for the GCD to be up depending on your server lag you can sometimes avoid that error.

Personally if I need to move more than a couple a spaces I alternate DP and SW: D to avoid that issue all together.